1. Field of the Invention
This invention relates to a process for the preparation of tertiary olefins from the corresponding tertiary alkyl ethers, and more particularly, to such process using an improved catalyst therein.
2. Description of the Prior Art
Tertiary olefins are important starting materials for the preparation of polymers and specialty chemicals. For example, isobutylene is used as a monomer for making elastomers such as polyisobutylene and butyl rubber, and as an intermediate for the production of isoprene, diisobutylene, tert-butyl phenol, tertbutyl amine, tert-butyl mercaptan and methyl methacrylate. Tertiary olefins usually are obtained by decomposing the corresponding tertiary alkyl ethers. For example, decomposition of methyl tertiary butyl ether (MTBE) provides isobutylene, while decomposition of tertiary amyl methyl ether (TAME) will produce isoamylene.
Several processes have been proposed for preparing tertiary olefins from tertiary alkyl ethers. For example, U.S. Pat. Nos. 4,447,668, 4,551,567, 4,570,026 and 4,751,343 disclose a process using a cation acidic exchange resin as catalyst; Japanese Patent Publication 41,882/72 discloses a process in which MTBE is decomposed using a alumina-type acidic solid catalyst having a specific surface area of at least 25 m.sup.2 /g; U.S. Pat. No. 4,006,198 describes a process using a catalyst composed of active alumina modified by reaction with a silicon compound; Japanese Patent Application Laid-Open Specification 2,695/80 discloses a process using a catalyst comprising silica combined with various metal oxides; Japanese Patent Application Laid-Open Specification 94,602/74 proposes a process using an active carbon catalyst; U.S. Pat. No. 3,637 889 teaches a process using a metal sulfate as a catalyst; U.S. Pat. No. 4,398,051 describes a process using a catalyst produced by calcining an aluminum compound supported on a carrier containing silicon oxides at a temperature above that at which the aluminum compound decomposes; U.S. Pat. No. 4,395,580 describes a process in which the decomposition is carried out in the presence of steam by using a catalyst comprising titanium, hafnium or zirconium supported on alumina; Japanese Patent Application Laid-Open Specification 13,734/84 describes a process in which the decomposition is carried out in the presence of steam and the catalyst comprises heteropoly acids on neutral or acidic supports; European Patent Application 118,085 relates a process using a solid phosphoric acid catalyst which has been calcined at a temperature higher than 500.degree. C. in an inert gas; Japanese Patent Application Laid-Open Specification 106,031/87 teaches a process using niobic acid as catalyst; Japanese Patent Application Laid-Open Specification 53,739/90 teaches a process using B-containing porous glass as catalyst; Japanese Application Laid-Open Specification 53,740/90 teaches a process using tantalic acid as catalyst; U.S. Pat. No. 4,343,959 discloses a process using a catalyst obtained by calcining a silica-alumina compound at 700 to 1000.degree. C.; U.S. Pat. No. 4,254,296 discloses a process using a catalyst system which consists of a crystalline silica having a high specific surface area which is modified with an oxide of a metallic cation; U.S. Pat. No. 4,371,725 discloses a process which is carried out in the presence of hydrogen using an acidic molecular sieve as catalyst.
These disclosed processes, however, are not entirely satisfactory for practical industrial operations for the following reasons. (1) A dimethyl ether by-product may be formed as a dehydration of two molecules of methanol itself the product of the decomposition of MTBE; accordingly the amount of alcohol recovered during the process is low. (2) Furthermore, the tertiary olefin product may dimerize, or even trimerize, reducing the yield of product. (3) The olefin also may be hydrated to the corresponding tertiary alcohol. (4) A high reaction temperature may be required; (5) The preparation of the catalyst used may be exceedingly complex, or require expensive chemicals; and (6) moreover, the catalyst life may be short and/or unstable toward steam, and/or high temperatures used during the process.
Accordingly, it is an object of this invention to provide an industrial process for the preparation of tertiary olefins by the decomposition of tertiary alkyl ethers at a high conversion and selectivity of products obtained using an improved catalyst composition which can function effectively at relatively low reaction temperatures and is stable and durable.